1. Field of the Invention
The present invention relates to windshear data processing. More particularly, the invention relates to formation of a single or multiple microburst downdraft candidates and calculation of horizontal outflow velocities for microburst downdrafts.
2. Description of the Related Art
A microburst is a meteorological phenomenon of atmospheric instability that causes a windshear, which is hazardous to aircraft, especially at low altitudes, for example, during final approach and initial take-off. The instability is created by warm air at lower altitudes supporting cooler, dense air at higher altitudes. When the cooler air mass exceeds a variable threshold, warm air can no longer support the cold air, and the cold air begins to penetrate and descend, often accelerating further due to evaporative cooling. A microburst derives its name from the column of cold air that descends and spreads out in all directions upon impacting with the ground. Such spreading is governed by fluid dynamics and the laws of conservation of mass. Behaving almost as an incompressible fluid, the columnar downdraft to the ground is transformed into radial outflow along the ground, producing an upside-down mushroom-like effect.
For aircraft at low altitudes, these windshear outflows are hazardous because they create a region where the local wind changes from a headwind to a tailwind. Aerodynamically, this is a performance decreasing windshear and will cause an aircraft to lose altitude. The hazard is exacerbated by the downdraft and the initial impression of the performance enhancing headwind. The severity of the hazard is balanced by the aircraft's ability to initiate and sustain altitude performance, that is, its thrust-to-weight ratio. The severity presented to an aircraft by a microburst is determined by the "total hazard factor," which is comprised of two physical contributions, namely, a horizontal term relating to the spatial rate of wind change (i.e., headwinds changing to tailwinds) and a vertical term due to downdraft.
Conventionally, a double elevation or bar radar scan transmitted from the ground has been used to detect hazardous weather. When a microburst is detected, the pilot is alerted before physical contact with the microburst by the personnel monitoring the ground based radar. Using such a double bar scan, relatively large vertical fan beams overlap and produce an upper beam Doppler spectrum and a lower beam Doppler spectrum, subtracted to allow the velocity bound of the difference Doppler spectrum to provide wind speed components at heights near the surface.
This ground based approach, however, is not particularly applicable to airborne systems having scans looking downward, because airborne radar typically employs higher frequencies, maximal directive gain, and minimal antenna sidelobes. Such a double overlapping beam scan is described in U.S. Pat. No. RE 33,152. Airborne turbulence mapping systems typically involve pulse pair processing approaches, in which autocorrelation is used between the echoes, as typified by U.S. Pat. No. 4,835,536. Alternatively, the turbulence can be characterized by a variance of velocities over a range, without respect to a performance decreasing structure, as described in U.S. Pat. No. 4,223,309. Neither of the latter two approaches, however, is suitable for a downlooking system for detecting windshear.
A system has been described that aids pilots in avoiding windshear hazards and that also minimizes false windshear alarms. This system is described in U.S. patent application Ser. No. 07/714,133, filed Jun. 13, 1991, and assigned to the Assignee of the present invention, for which a Notice of Allowability issued on Oct. 6, 1992. This system employs a two-elevation bar scan to produce horizontal wind maps at differing altitudes. The upper bar scan indicates the presence of a Doppler structure for microbursts at antenna pointing angles that minimize the illumination of vehicular targets moving along the ground. Using the microburst downdraft candidates identified by the upper bar scan, a total windshear hazard factor can be constructed and/or detected. If, however, no microburst downdraft candidates are identified by the upper bar scan, no total hazard factor detection and/or construction will occur.
Using this system, microburst downdraft candidates are recognized by an expansive and continuous velocity ordered structure existing with angular continuity, establishing a candidate by the consistency of velocity ordering over an angle swath. For successive azimuth angle lines-of-site, the range of peak headwind (e.g., the least negative Doppler) must either consistently lie at greater or lesser ranges than the range of peak tailwind (e.g., the least positive Doppler). This system defined runs of velocity ordering by how many M of the last N order angle reports were of the initiating, dominant ordering (M less than or equal to N). A run will furnish a candidate record when the number of same ordered angle reports exceeds a certain threshold level. When a run fails the continuation criteria, a new run is begun where the prior run of angle reports stops. When the number of reporting angles is large enough, a candidate record is written which includes the range and azimuth angle to peak headwind and peak tailwind for each line-of-site of the azimuth angle run.
This system for candidate formation employing an "adaptive M of N type continuity algorithm" is not highly accurate, however, in determining microburst downdraft candidate angle location. Moreover, this system suffers from an inability to efficiently form candidates from small strings of angular data reports (i.e., early detection of approaching microburst downdrafts). Finally, this system is capable of determining at most a single candidate for any one line-of-sight of the radar. This is because this system only searches for global maximum and minimum Doppler velocities along each angular line-of-sight scan by the radar. Thus, this system lacks the capacity to identify and form multiple candidates in range.
The two bar radar scan system described in U.S. patent application Ser. No. 07/714,133 (cited above) also furnishes information about the change in horizontal outflow velocity of a microburst downdraft as a function of altitude above the ground. In that system, the horizontal outflow of velocity at either elevation scan bar for a sensed microburst candidate region was calculated by assuming a circularly-symmetric outflow in space. Given the centered location in range and angular coordinates of a downdraft candidate, the peak head and tailwinds along any radar line-of-site within the candidate outflow velocity locus could be compensated for the Doppler projection of the viewing geometry. In addition, an estimate for the average horizontal outflow over the outflow circumference could be formed by calculating the incrementally summed flux (i.e., radial velocity multiplied by the incremental circumference) at each point along the locus normalized by the circumference of the assumed circle. The radius of the assumed circle was established by an amplitude weighted mean and ratio standard deviation of the amplitude data reported within the candidate locus. Once the horizontal outflow velocity (i.e., the horizontal term of the total hazard factor) was calculated, the vertical downdraft term could be deduced, in general, from the mass continuity principle.
Assuming the velocity outflow is circularly symmetric, an algorithm assuming circular symmetry is limited in accuracy only by random processes. If the velocity outflow is non-circularly symmetric, however, an algorithm assuming circular symmetry is additionally inaccurate, because the projection of the winds along the radar line-of-site are compensated using a circular outflow locus. Using such a circularly symmetric model, therefore, errors will arise in the presence of asymmetric outflows, analogous to the errors that arise in using orthogonal functions. That is, a linear function modeling a quadratic function can approximate much of the quadratic data, but it is fundamentally incapable of achieving high accuracy.
Although most microbursts are circularly symmetric, asymmetric microbursts will result from a non-normal impact. That is, a microburst downdraft in the presence of a strong prevailing/steering wind will not descend vertically, but will have slightly non-perpendicular impact with the ground, which will produce some spatial and velocity asymmetry. Furthermore, multiple clustered microbursts, a.k.a. "lines," can occur, which can also produce some spatial and velocity asymmetry. A model assuming circular symmetry on non-circularly symmetric data, however, will cause errors. Thus, a higher order, non-circularly symmetric model will limit the inaccuracies arising from a circularly symmetric model.
Conventional systems, including the system described in U.S. patent application Ser. No. 07/714,133, frequently fail to account for the presence of microbursts having a relatively small radius and outflow. As a result, the total hazard factor will be underestimated, because calculation of total hazard factor does not reflect these small radii events.
Accordingly, a need exists for a system for achieving more accurate microburst downdraft candidate angle location and expanse with superior abilities to efficiently and quickly detect approaching microburst downdraft candidates and for identifying multiple candidates in range. Moreover, a need exists for a system for determining the horizontal term, and calculating therefrom the vertical term, of a total hazard factor resulting from a microburst downdraft candidate having asymmetric qualities and for accurately detecting small radii microbursts.